Dielectric heating involves the heating of materials by dielectric loss. A changing electric field across the dielectric material (in this case, a load of clothes) causes energy to be dissipated as the molecules attempt to line up with the continuously changing electric field, creating friction. This changing electric field may be caused by an electromagnetic wave propagating in free space as in a microwave oven, or it may be caused by a rapidly alternating electric field inside a capacitor, as in the present invention. In the latter case, there is no freely propagating electromagnetic wave. This changing electric field may be seen as analogous to the electrical component of an antenna near field.
Frequencies in the RF range of 1 MHz to 50 MHz have been used to cause efficient dielectric heating in some materials, especially liquid solutions with polar salts dissolved. These relatively low frequencies can have significantly better heating effects than higher, e.g., microwave frequencies, due to the physical heating mechanisms. For example, in conductive liquids such as salt water, “ion drag” from using lower RF frequencies causes heating, as charged ions are “dragged” more slowly back and forth in the liquid under influence of the electric field, striking liquid molecules in the process and transferring kinetic energy to them, which is eventually translated into molecular vibrations, and thus into thermal energy.
Dielectric heating at these low frequencies, as a near-field effect, requires a distance from the radiator to the absorber of less than about 1/16th of a wavelength (λ) of the source frequency. It is thus a contact process or near-contact process, since it usually sandwiches the material to be heated (usually a non-metal) between metal plates that set up to form what is effectively a very large capacitor, with the material to be heated acting as a dielectric inside the capacitor. Actual electrical contact between the capacitor plates and the dielectric material is not necessary, as the electrical fields that form inside the plates are what cause the heating of the dielectric material. However, the efficient transfer of the RF heating energy to the load is greatly improved as the air gap that may arise between the capacitor plates and the load is minimized.
At higher frequencies, e.g., microwave frequencies>800 MHz, the wavelength of the electromagnetic field becomes closer to the distance between the metal walls of the heating cavity, or to the dimensions of the walls themselves. This is the case inside the cavity of a microwave oven. In such cases, conventional far-field electromagnetic (EM) waves form; and the enclosure no longer acts as a pure capacitor, but rather as a resonant cavity. The EM waves are absorbed into the load to cause heating. The dipole-rotation mechanism of induced heat generation remains the same as in the case of capacitive electrical coupling. However, microwave induced ion rotation is not as efficient at causing the heating effects as the lower RF frequency fields that depend on slower molecular motion, such as those caused by ion drag.
Novel applications of RF dielectric heating to the drying of clothes have been patented in commonly owned U.S. Pat. Nos. 8,826,561 and 8,943,705, where rotary RF heating capacitive structures are disclosed. These patented inventions require the introduction of specialized connections to both anodes inside the dryer drum and to the drum surface acting as a cathode.